As generally known, in an MR imaging (MRI) system or MR scanner, an examination object, usually a patient, is exposed to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object form a certain net magnetization of all nuclei parallel to the B0 field, which can be tilted leading to a rotation around the axis of the applied B0 field (Larmor precession). The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei, namely their gyromagnetic ratio, and the strength of the applied B0 field. The gyromagnetic ratio is the ratio between the magnetic moment and the spin of a nucleus.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state, wherein T1 and T2 is the time required for the longitudinal and transversal magnetization, respectively, to return to 63% of its equilibrium value. MR signals which are generated by the precessing magnetization, are detected by means of an RF receive antenna or coil. The received MR signals which are time-based amplitude signals, are then Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object.
In order to obtain a spatial selection of a slice or volume within the examination object and a spatial encoding of the received MR signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions. Due to the fact that the Larmor frequency is dependent on the strength of the magnetic field which is imposed on the nuclei, the Larmor frequency of the nuclei accordingly decreases along and with the decreasing gradient (and vice versa) of the total, superimposed B0 field, so that by appropriately tuning the frequency of the transmitted RF excitation pulse (and by accordingly tuning the resonance frequency of the RF/MR receive antenna), and by accordingly controlling the gradient magnetic fields, a selection of nuclei within a slice at a certain location along each gradient in the x-, y- and z-direction, and by this, in total, within a certain voxel of the object can be obtained.
Medical instruments or devices especially in the form of interventional and non-interventional instruments, tools and other as mentioned above are frequently used during the examination or treatment of an examination object and especially of a local zone or area thereof by means of an MR imaging apparatus. Such medical instruments or devices are for example pacemakers, catheters, biopsy needles, surgical devices, pointers and other which are used for example for biopsies, thermal ablations, brachytherapy, slice selection and other invasive or non-invasive purposes as mentioned above. Further, RF surface coils, RF pad coils, RF head coils, stereotactic frames and other non-interventional instruments are also used during MR imaging. For all these and other examinations it is important to precisely position the instrument and especially a certain part or property thereof (like its tip or axis) at a certain desired location at or within the examination object. This requires that during the positioning of the instrument by an operator, the current position of the instrument or an interesting part thereof, especially its tip, is precisely determined and imaged or indicated in the MR image of the related examination object, so that a desired destination at or within the examination object can be reached.
For this purpose, the above instruments or medical devices can be equipped with a position marker having a local RF coil, the position of which can be imaged by means of an MR imaging apparatus in the MR image of the related examination object.
A desired spot-like indication of the position and by this a desired accuracy of the position indication can be obtained either by dimensioning the local RF coil such small that it receives (“local”) MR signals substantially only (but with a sufficient MR signal strength) from an accordingly small or spot-like local volume of the examination object, and/or by providing a small or spot-like local volume in the form of a marker material (e.g. 19F, 13C, 23Na or other) having a gyromagnetic ratio and accordingly a Larmor frequency which is different from the gyromagnetic ratio and the Larmor frequency of the material of the examination object (usually water and fat), so that upon RF excitation of this local volume only, the excited (“local”) MR signals provide a spot-like signal source which can be imaged in an MR image of an examination object. In the latter case, more in detail, by a first RF pulse sequence the position data of the marker material is determined and by a second RF pulse sequence the image data of the examination object is determined, and then both data sets are displayed in the form of a common MR image.
Generally, two different types of such position markers can be distinguished, namely active and passive markers. Active markers as defined above usually comprise a sensor especially in the form of a local RF coil for receiving the said local MR signals emitted from a local volume, wherein these local MR signals are conveyed by means of a cable to a remote MR receiver of an MR imaging apparatus in order to determine and/or image the position of the local volume and by this the position of the marker on the basis of the received local MR signal as explained above.
In contrast to this, passive markers are usually imaged in an MR image for example by distorting, enhancing or modifying due to their physical properties or due to an own (intrinsic) RF resonance (which is excited by the applied external RF excitation field), the B0 field or the RF excitation field transmitted by the MR imaging apparatus and by this the MR signals emitted by the examination object.
All these principles enable a position determination and visualization of the active (and passive) marker, respectively, in connection with the applied gradient magnet fields within the MR image of an examination object as explained above.
However, one major drawback of the above active markers and of interventional or non-interventional instruments comprising such an active marker is, that an RF cable connection is required for feeding the received MR signals from the active marker to a remote MR receiver or MR imaging apparatus. On the one hand, such a cable reduces the comfort and ease of use and introduces mechanical safety risks, especially in case of an interventional instrument, limits the flexibility of handling, and increases the time needed to prepare an MR imaging procedure. On the other hand, a (metallic) cable for connecting the active position marker with an MR receiver usually has to be guided inside and through an examination space of an MR imaging apparatus, so that it poses a potential safety risk due to resonant common mode currents which are induced on the cable by the RF excitation field emitted by the related RF transmit antenna of the MR imaging apparatus.
WO 2006/103635 discloses an interventional device which is connected by means of a cable to a remote spectrometer, for conducting an informative signal from the interventional device to the spectrometer. Common mode resonances on the cable are avoided by subdividing the cable into a plurality of capacitively coupled portions of the cable. The resulting attenuation of the informative signal is compensated by amplifying the signal by means of a parametric amplifier included into the interventional device, wherein a pump signal is supplied from the spectrometer to the parametric amplifier over the cable for converting the frequency of the informative signal to a higher frequency which is subject to a lower attenuation on the cable.